NEG-EXISTS-ELIM-DUP-TAC : TACTICNEG-EXISTS-ELIM-DUP-TAC is a tactic.
As defined for use ETREE-NAT: is a primitive tactic.
If planned line is FALSEHOOD, and a support line is a negated existentially
quantified formula with more than one expansion, one of which is admissible,
applies eneg rule, adding the line with its other expansions as a support.
Same as Pfenning's tactic 221.
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